Setup

Then insert the 2 transparent elements into the 2 holes near the micro USB socket.
(Yes, there are 2 plugs for 4 holes and no instructions.)

Now insert the Raspberry Pi and then insert the SD card.
The contacts should face upwards.
(It is near impossible to the the cards out again.)

After switching it on, you can connect to it via http://octopi.local .

SSH

You can also access the raspberry via SSH

ssh pi@octopi.local

The default password is "raspberry.
The SD card is mounted as /boot
The OctoPrint config file is at "/home/pi/.octoprint/config.yaml"
You can restart the server via "sudo /etc/init.d/octoprint restart"

If your Wifi access point via

sudo sudo iwlist wlan0 scanning | grep ESSID

can't be seen by Linux, run

sudo raspi-config

and select "5 internationalization options" -> "I4 select Wifi locale"
to enable the Raspberry to see all Wifi channels that are legal in your country.

The Raspian I got was very old. I had to provide Internet via Ethernet and do

sudo apt-get update
sudo apt-get dist-upgrade

then it was able to see Wifi networks on Channel 40 (5GHz) and 12+13 (2.4GHz).

GPIO fun

While at the shell, you can have fun with the GPIO pins in Bash.
Sadly you can't set the pull-up resistors from the shell.
However my image came with WiringPi already installed.
It doesn't have a "--help" or a man page on the Pi itself, so here are the basics:

gpio readall

gpio mode (pin) in/out

gpio mode (pin) up/down/tri (set pull up resistors)

gpio read (pin)

gpio write (pin) 0/1

gpio wfi (pin) rising/falling/both (non-busy waiting for a state change)

Cura slicing

Luckily the Bundle comes with thje CuraEngine plugin preinstalled. So slicing it not much of a problem.You can imort your existing 15.x profiles (but not 2.1.1 profiles) in Settings->Plugins->CuraEngine->import profile.

Cura 2.1

The documentation should be here However that's not the whole picture incomplete.
You need an Ultimaker2extended, Ultimaker2Go or Ultimaker2 profile with the reprap g-code flavor to have start and end added to your gcode files including material temperatures, homing and shutdown. Like this one.
To avoid adding files to Cura itself (and keeping them after updating Cura),
you can put your .json files for a new machine definition here:

Due to bug #850, you need to copy the fdmprinter.json, Ultimaker2.json and other files you inherit from into the same directory.

You can also add additional materials (use the existing materials in Cura 2.1.2.app/Contents/Resources/cura/resources/profiles/materials as a reference) to
~/.cura/profiles/materials (OSX)
but
be careful, the file structure is identical to the MATERIALS.txt that
the firmware imports from an SD card but the property and section names
inside are different. Strange design decision.

Ultimaker II attachment

mechanical

It looks like self adhesive Velcro is the best option to attach the box to the back of your Ultimaker II.

Electrical

Having a Raspberry Pi permanently connected to your printer, that has ample 5V and 12V, it is kind of silly to power it via a separate wall wart. So we should see about powering it from the Ultimaker.